As used herein, xe2x80x9cautomotive refinishxe2x80x9d refers to compositions and processes used in the repair of a damaged automotive finish, usually an OEM provided finish. Refinish operations may involve the repair of one or more outer coating layers, the repair or replacement of entire automotive body components, or a combination of both. The terms xe2x80x9crefinish coatingxe2x80x9d or xe2x80x9crepair coatingxe2x80x9d may be used interchangeably.
Automotive refinishers must be prepared to paint a wide variety of materials. Examples of commonly encountered materials are one or more previously applied coatings, plastic substrates such as RIM, SMC and the like, and metal substrates such as aluminum, galvanized steel, and cold rolled steel. Bare metal and plastic substrates are often exposed as a result of the removal of the previously applied coating layers containing and/or surrounding the defect area. However, it is often difficult to obtain adequate adhesion of refinish coatings applied directly to exposed bare substrates.
Among the many factors influencing the degree of refinish coating/substrate adhesion are the type of exposed substrate, the presence or absence of adhesion promoting pretreatments and/or primers, the size of the exposed area to be repaired, and whether previously applied xe2x80x9canchoringxe2x80x9d coating layers surround the exposed repair area.
For example, refinish adhesion is particularly challenging when the exposed substrate is a bare metal such as galvanized iron or steel, aluminum or cold rolled steel. It is especially hard to obtain adequate refinish adhesion to galvanized iron. xe2x80x9cGalvanized iron or steelxe2x80x9d as used herein refers to iron or steel coated with zinc. xe2x80x9cSteelxe2x80x9d as used herein refers to alloys of iron with carbon or metals such as manganese, nickel, copper, chromium, molybdenum, vanadium, tungsten and cobalt.
Refinish operations have traditionally used adhesion pretreatments to overcome the adhesion problems associated with the coating of bare metal substrates. Pretreatment as used herein may refer to either mechanical or chemical alterations of the bare metal substrate. Mechanical alterations used to obtain improved adhesion include sanding, scuffing, and the like. Chemical alterations include treatment of the substrate with compositions such as chromic acid conversion coatings, acid etch primers and the like.
Although such pretreatments have obtained improved refinish adhesion, they are undesirable for a number of reasons. Most importantly, pretreatments are inefficient and expensive to apply in terms of material, time, and/or labor costs. Some chemical pretreatments also present industrial hygiene and disposal issues. Finally, the use of some pretreatments such as acid etch primers may contribute to water sensitivity and/or coating failure under test conditions of extreme humidity.
Accordingly, it is highly desirable to eliminate the need for substrate pretreatment as regards the refinish coating of bare metal substrates.
In addition, adhesion to bare metal substrates is improved when the defect area to be repaired is relatively small and is surrounded by previously applied coating layers. Such previously applied coating layers act as an xe2x80x98adhesion anchorxe2x80x99 to the refinish coating. However, many refinish repairs are of a size such that they lack any surrounding adhesion anchors. Moreover, such anchoring adhesion may be completely absent when replacement body parts are painted with a refinish coating.
Finally, improvements in refinish adhesion to bare exposed metal substrates must not be obtained at the expense of traditional refinish coating properties. Such properties include sandability, recoatability, corrosion resistance, durability, ambient or low temperature cure, application parameters such as pot life, sprayability, and clean up, and appearance. Performance properties such as sandability, recoatability and corrosion resistance are particularly important for coating compositions intended for use as primers over steel substrates.
However, it has been difficult for the prior art to obtain the proper balance with regard to sandability, recoatability, corrosion resistance, and metal adhesion requirements.
Failure to provide adequate corrosion resistance or salt spray resistance typically manifests as xe2x80x9cscribe creepxe2x80x9d. xe2x80x9cScribe creepxe2x80x9d refers to the degree of corrosion and/or loss of adhesion which occurs along and underneath film adjacent to a scribe made in a cured film after the scribed film has been placed in a salt spray test apparatus. The scribe generally extends down through the film to the underlying metal substrate. As used herein, both xe2x80x98corrosion resistancexe2x80x99 and xe2x80x98salt spray resistancexe2x80x99 refer to the ability of a cured film to stop the progression of corrosion and/or loss of adhesion along a scribe line placed in a salt spray test apparatus for a specified time. Cured films that fail to provide adequate salt spray resistance are vulnerable to large scale film damage and/or loss of adhesion as a result of small or initially minor chips, cuts and scratches to the film and subsequent exposure to outdoor weathering elements.
Although urethane coatings have been known to be useful as refinish primers, they have not achieved the desired balance of properties.
In particular, for polyurethane films to provide desirable salt spray resistance, they have typically relied upon the use of corrosion protection components containing heavy metal pigments such as strontium chromate, lead silica chromate, and the like. Unfortunately, sanding such a film produces dust that is environmentally disfavored due to the presence of the heavy metal containing pigments. Since sanding is a necessity for automotive refinish primers, this disadvantage can render the coating unusable in most commercial refinish application facilities. Accordingly, it would be advantageous to provide a coating which can provide adequate salt spray resistance but which is substantially free of any heavy metal containing pigments.
Aluminum pigments have traditionally been used to provide a desirable metallic or lustrous appearance. For example, the 1977 Federation Series on Coatings Technology teaches that aluminum pigment containing paints have no specific anti-corrosive effect, such as is afforded by rust-inhibitive pigments traditionally used in commercially acceptable metal primers. Indeed, it is further taught that strontium chromate should be used in combination with aluminum pigments to provide aluminum containing paints having an anti-corrosive effect.
Aluminum pigments, especially leafing aluminums, are known to produce an apparently continuous film of aluminum metal.
Barrier pigments, especially platy or platelet pigments have been known to provide anticorrosive effects.
However, leafing aluminums and barrier pigments have traditionally been somewhat disfavored due to recoatability and/or sanding performance issues. Moreover, the anticorrosive effect of the coating post sanding can be impaired due to the removal of the barrier or leafing layer. As a result, the use of aluminum pigments in primers is to some extent disfavored.
The prior art has thus failed to provide a coating composition intended for use as a direct to metal primer which has commercially acceptable performance properties with regard to salt spray resistance, sandability, recoatability and adhesion to metal substrates, especially iron and/or steel.
Accordingly, it is an object of the invention to provide a curable coating composition that can be applied directly to a metal substrate and provides a commercially acceptable level of salt spray resistance.
It is a further object of the invention to provide a curable coating composition which has commercially acceptable performance properties with regard to direct to metal adhesion and salt spray resistance and further can be sanded without the production of environmentally disfavored dust.
It is a further object of the invention to provide a curable coating composition which has commercially acceptable performance properties with regard to direct to metal adhesion, salt spray resistance, sandability, and further can be recoated with a second application of the curable coating composition of the invention or another curable coating composition.
Finally, it is an object of the invention to provide a curable coating composition which has commercially acceptable performance properties with regard to direct to metal adhesion, salt spray resistance, sandability, and recoatability, especially a curable coating composition having a film forming component selected from the group consisting of polyurethane systems and epoxy/amine systems.
It has been found that these and other objects of the invention have been achieved with the use of a curable coating composition comprising a film-forming component selected from the group consisting of polyurethane systems and epoxy/amine systems, and a corrosion protection component consisting of aluminum selected from the group consisting of nonleafing aluminum pigments and present in an amount effective to prevent corrosion of the substrate, wherein a cured film of the coating applied to a metallic substrate has a pass rating after 480 hours in salt spray per ASTM B117, and is both sandable and recoatable.
In a preferred embodiment of the invention, the aluminum pigment will be a lamellar shaped aluminum pigment and will be present in an amount of from 0.011 to 0.051 P/B.
In a particularly preferred embodiment of the invention, the film forming component of the invention will be a polyurethane based coating system comprising a film forming polymer which is an active hydrogen containing group polymer and an isocyanate functional crosslinking agent.
In a most preferred embodiment of the invention, the polyurethane film forming component will further comprise a composition comprising (I) an effective amount of a first compound having an acid number of from 70 to 120 mg KOH/g, a hydroxyl number of from 200 to 400 mg KOH/g, a number average molecular weight of from 300 to 700, and which is the reaction product of (a) at least one difunctional carboxylic acid, (b) at least one trifunctional polyol, (c) at least one chain stopper, and (d) phosphoric acid, and (II) an effective amount of a second compound comprising a carboxy phosphate ester having the formula: 
wherein R is an C5-C40 aliphatic group in which one or more aliphatic carbon atoms are substituted with lateral or terminal xe2x80x94COOR1 groups, wherein R1 is H, metal, ammonium, C1-C6 alkyl, or C6-C10 aryl, M is hydrogen, metal or ammonium and x is a number from 0 to 3.